1. Field of the Invention
This invention relates generally to high resolution spectrometer optics. More specifically the invention relates to diffraction gratings used in monochromators and spectrometers. Additionally the invention relates to a novel optical system for monochromators and spectrometers.
2. Description of Related Art
Currently monochromators used in the extreme ultraviolet (EUV) region have a resolving power, .lambda./.DELTA..lambda., of between 10.sup.2 and 10.sup.4. However, most synchrotron experiments require a .lambda./.DELTA..lambda. of 10.sup.3 -10.sup.5. However, there have been only a few efforts made to develop a high resolution monochromator that could be used for EUV synchrotron radiation. Such monochromators require improved grating designs and/or improved monochromator design in order to meet the requirements of this use.
Hideyuki Noda and Masato Koike disclosed a holographic diffraction grating made from a fringe pattern produced by the interference of two coherent light beams with at least one of the beams having an off-axis orientation and an astigmatic wavefront (U.S. Pat. No. 5,052,766). Usually holographic gratings are limited to the case of spherical and/or plane wavefront recordings. Noda and Koike's holographic grating had variable line spacings to reduce optical aberrations. The manner in which the relative location of grooves could be determined was left to trial and error.
Koike described a significant and successful method to improve the resolving power of monochromators by introducing a varied spacing diffraction grating having mechanically ruled grooves with varied spacing between them (Ser. No. 08/277,404; notice of allowance received Nov. 29, 1995; issue fee paid Feb. 22, 1996, U.S. Pat. No. 5,528,364). The groove spacing was determined using a hybrid design method. Groove spacing was approximately determined by a formula d.sub.n =d.sub.0 +2an+6bn.sup.2 +4cn.sup.3, where d.sub.n was the approximate spacing between the nth groove and the (n+1) groove and d.sub.0 =d+a-b where d was the effective grating constant; the ruling parameters 2a, 6b, and 4c were determined from the resolving power R for a specific optical configuration. This method eliminated a lot of the trial and error previously used in making varied-line-spacing gratings. The diffraction grating disclosed by Koike had two difficulties associated with it. The mechanical etching left the grooves with imperfect optical edges, which introduce anomalies into the resulting optical signal; and the parameters 2a, 6b, and 4c, must be uniquely determined for each specific optical configuration.
Due to the small source-point size of third-generation synchrotron radiation sources, high throughput can be made compatible with high resolution simply by eliminating the entrance slit. This requires thermal stability for the grating which in turn requires grating material like silicon carbide (SiC) . Currently mechanically ruled SiC gratings require a coating of gold or some other suitable metal on the blanks in order to etch the rulings, and under high heat those metal coatings peel.
It is highly desirable also to suppress overlapping higher orders and scattered light in addition to the aberrations. A laminar grating with a proper land-width to period ratio provides even-order suppression at normal and near-normal incidence. Furthermore, a low stray light level is expected from a holographic grating, both blazed and laminar, because the slow slopes of grooves blazed by preferential etching in Silicon and the lands of square grooves formed in SiC by reactive ion-beam etching are smoother than those produced by mechanical ruling.
These considerations all indicate the need for further studies of holographic gratings for high-resolution and high-flux EUV grazing incidence monochromators.
It would be very useful and desirable to have a diffraction grating that did not contain the imperfections on the groove edges that result from mechanical etching. It would be additionally desirable to be able to use holographic techniques to make diffraction gratings having variable groove spacings without having to resort to a "trial and error" method. It would be yet additionally desirable to be able to determine the groove spacings needed for a high resolution diffraction grating without having to perform a series of laborious numerical determinations, using ray tracing and the resolving power R to obtain ruling parameters 2a, 6b, and 4c for each specific optical configuration. It would be further desirable to be able to adjust a monochromator to optimize performance for a given grating, rather than to design and manufacture gratings that optimize a set optical configuration specific to a particular monochromator.